1. Field of the Invention
Coal represents approximately 84 percent of the known recoverable fossil fuel resources in the United States, while petroleum and natural gas account for about 10 percent and shale oil for the remaining 6 percent.
The shrinking petroleum and natural gas reserves around the world, and recently enacted governmental sulfur oxides emission standards in the United States, have highlighted the need to focus attention on coal desulfurization and conversion technology and the production of synthetic fuels therefrom to meet the growing demand for energy in the United States and around the world.
The sulfur content of coals ranges from approximately 0.2 percent to as much as 7 percent as mined. Sulfur does not occur as an element in coal, but three sulfur forms in chemical combination are recognized. Most of the sulfur, approximately 40 to 80 percent, occurs as a constituent of pyrite or marcasite. These polysulfides are deposited as famboids in peat and are formed by the reaction between ferrous ions and hydrogen sulfide which is derived from the action of anerobic bacteria on sulfates. The remainder of the sulfur occurs as hydrous ferrous sulfate (FeSO.sub.4.7H.sub.2 O) derived by weathering of pyrites, as gypsum (CaSO.sub.4.2H.sub.2 O), and as organic sulfur in combination with coal-formed vegetal material (macerals). The amount of sulfate sulfur is very small, generally less than about 0.1 percent. Organic sulfur is presumed to exist as sulfides, disulfides, mercaptans, and heterocyclic rings, for example thiophene.
Sulfur has always been an objectionable constituent of coal and other carbonaceous materials normally used to generate power and energy. Combustion of coal or other carbonaceous materials is by far the largest single source of atmospheric sulfur dioxide pollution in the United States, accounting for approximately two-thirds of the total sulfur oxide emissions.
With the rapidly diminishing oil supply and the present world-wide emphasis on the energy crisis, increased attention by both government and private organizations is being given to coal and synthetic fuels produced therefrom as an alternate energy source. However, depending upon their origin, coals contain varying amounts of sulfur in the form of pyrite, organic sulfur and sulfate sulfur from which sulfur dioxide is formed when coal, and synthetic fuels produced from coal are burned. Hence, there is an ongoing search to convert high sulfur content coal into a usable fuel which would meet current air quality standards and, additionally, help to aleviate the current shortage of crude oil. The best processes which are known to the art, generally do not combine a process for desulfurizing coal or carbonaceous materials and producing a synthetic fuel.
2. Brief Description of the Prior Art
The problem of separating sulfur and other impurities and producing synthetic fuels from coal and carbonaceous materials is known and several processes have been extensively tested over the years. One such process is set forth in U.S. Pat. No. 3,926,575 which discloses a process for reducing the pyritic content of coal, which comprises reacting finely divided coal with sulfuric or hydrochloric acid. An alternate method is disclosed for removing sulfur from coal which involves, in addition to an acid leach, a final extraction step involving use of a solvent, for example, benzene, etc.
Another process is disclosed in U.S. Pat. No. 4,071,328 which relates to a method of removing sulfur from coal which consists of hydrogenating the coal to remove a portion of sulfur therefrom. Next, the hydrogenated coal is contacted with an aqueous inorganic acid solution to remove some of the sulfur initially combined as pyritic sulfur.
U.S. Pat. No. 4,054,421 relates to a process for reducing the sulfur content of chars made from coal, by washing the char with an acid followed by treatment with hydrogen gas. Particularly, coal is converted to char and thereafter introduced into an acid bath, comprising an aqueous solution of a mineral acid, for example hydrochloric acid.
A process for upgrading solid fuels is disclosed in U.S. Pat. No. 4,047,898 which teaches treatment of low rank fuels, for example, subbituminous coals with water under increased temperature and pressure in the presence of hydrogen. Particularly, the patent discloses a method for the benefication of low rank solid fuels which consists of mixing a low rank fuel with water, heating the mixture to a temperature above 300.degree. F. and at a pressure sufficient to maintain water in the liquid phase under reaction conditions, for a time period sufficient to reduce the sulfur and ash content and increase the fuel BTU value. As can readily be determined from the above, there is an ongoing search for new and more efficient processes for desulfurizing coal and carbonaceous materials and for producing useful synthetic fuels therefrom.